Different gpu without sli.how will it work in terms of memory
The cards are completely separate, they do not share memory. Whatever the lowest amount  of vram (in your case 8gb) will be the limit in blender.

The reason being that the whole scene is loaded onto each card. They are very likely rendering at different speeds, meaning your stronger card will be rendering more tiles then the weaker card.
(Think of it as if each graphics card is a computer in a render farm.)